1. Field of the Invention
The present invention relates to a pressure control apparatus for providing control over the fluid pressure of a gas or a liquid within a container or a flow passage, and to an apparatus employing the same, that is, an apparatus for effecting extraction, chromatographic separation and, fractionation which apparatus is arranged to extract a soluble substance from a sample by the use of a supercritical fluid or a liquefied gas, introduce the resultant extract into a chromatograph in an on-line manner for separation into individual components, and collect them as fractions.
2. Description of the Prior Art
The present inventor proposed an extraction/chromatographic separation apparatus capable of extracting a soluble substance from a sample by using a supercritical fluid or a liquefied gas and introducing the resultant extract into a chromatograph in an on-line manner, thereby separating the individual components of that extract (Journal of Chromatography, 332 (1985), pages 107 to 116).
The aforementioned extraction/chromatographic separation apparatus is arranged to deliver an extraction/elution fluid containing the supercritical fluid or the liquefied gas by means of a pump to the interior of an extraction vessel including a sample, extract a soluble component from the sample in the extraction vessel, cause the extract primarily obtained in a flow passage downstream of the extraction vessel to be drawn at a low pressure into a trapping loop previously charged with the extraction/elution fluid in the gaseous phase thereof, temporarily store that extract in that trapping loop, and introduce the extract to the chromatograph in an on-line manner.
A back pressure control valve for allowing the extract to be separated into its individual components in a separation column and for introducing the resultant components into a detector is disposed downstream of the detector which constitutes a part of the chromatograph.
The above-described apparatus accomplishes a substantial improvement in that extraction and separation can be effected in an on-line manner.
However, it is necessary to set the trapping loop apart from the extraction vessel and the separation column of the chromatograph, to connect the trapping loop to only one of the extraction vessel and the separation column, or to connect the trapping loop directly to the pump. Therefore, three six-way valves are needed, and this necessity disadvantageously complicates the construction and operation of the apparatus.
In general, typical prior art pressure control apparatus have been arranged to provide control so that a detection pressure may be set to a desired pressure by adjustment of spring force which is loaded on the pressure sensing piston, on which a value needle is installed so as to change the opening of a pressure control valve. Such a valve is commercially available from Tescom (MN).
However, since this pressure control valve is arranged to adjust the opening of the gap between a valve seat and a needle member by the movement of the pressure sensing piston, the volumes of the spaces defined upstream and downstream of that gap are relatively large and therefore the amount of fluid stagnating in the valve may reach at least several to several tens of milliliters.
As a result, even if separation is complete in the separation column, for example, in the previously-mentioned supercritical fluid chromatograph, an effluent may flow from the column into the pressure control valve to cause re-mixture of the separated components.
To the end of preventing such re-mixture, in a supercritical fluid chromatograph of the type employing a capillary column, a capillary tube having an inner diameter of 10 m or less is connected to a downstream side of the separation column so that the flow resistance of the supercritical fluid is utilized to maintain a required level of fluid pressure (Journal of Chromatographic Science, Vol. June 14, 1986, pages 236 to 241).
However, in order to vary the level of pressure, the flow rate of the fluid in the mobile phase must also be varied, with the result that the pressure and the flow rate both of which determine the period of retention time are varied at the same time. Therefore, it has been impossible to examine the details of the characteristics of the retention time by independently varying only one of the pressure and the flow rate.
It is known that a method of effecting separation under reduced pressure is widely used in supercritical fluid extraction apparatus. In this method, the level of pressure is lowered to cause a reduction in the solubility of an extract in the supercritical fluid, thereby separating and collecting a desired solute from the fluid.
However, if the aforesaid pressure control valve arranged to adjust the valve opening is used in a small supercritical fluid extraction apparatus, the following problems are encountered. Since a large amount of fluid stagnates in the pressure control valve, individual separated extracts are re-mixed within the pressure control valve, and this makes it impossible to fractionate the individual extracted substances. In addition, since the extracts stagnate in the valve when the pressure of the fluid, i.e., extraction medium, decreases rapidly causing solubility reduction, they are easily precipitated and adhered onto inner walls of the valve, thereby hindering the extracted substances from being completely fractionated. In addition, such precipitated substances often block the flow path of the valve.